Silica films are resistant to staining. Accordingly, there is a technology (see Patent Document 1) in which a coating liquid containing a polysilazane is coated onto a denture or the like and left in the air, the polysilazane reacts with moisture in the air, and a silica film is formed.
However, in the method recited in Patent Document 1, the speed of reaction is slow and it takes time to form (convert to) the silica film. Moreover, film properties of a silica film formed by reaction with moisture in the air are such that density is poor and the silica film is susceptible to damage.
Accordingly, there is a technology (see Patent Document 2) in which, when a polysilazane is being converted to a silica film, hydrogen peroxide is heated and vaporized and the vapor is blown onto a denture coated with the polysilazane, with the objective of making the reaction faster and improving product quality.
After the polysilazane is coated onto the denture, the base material is exposed to the high-temperature vapor containing hydrogen peroxide. As a result, the reaction converting the polysilazane to silica is accelerated by radicals produced from the hydrogen peroxide, and a uniform silica film is formed in a short time.
However, because the technology recited in Patent Document 2 employs high-temperature hydrogen peroxide, it is unsuitable for forming a silica film on a tooth in the mouth of a dental patient or the like.
A plasma can also be used in the conversion of a polysilazane to a silica film (see Patent Document 3). Plasma irradiation may shorten the duration of the conversion of the polysilazane to silica film without bringing the object being processed to a high temperature.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-286122    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-273913    Patent Document 3: Japanese Patent No. 5069582